A delusion is a fixed belief that is either false, fanciful, or derived from deception. It could be defined as a belief that is pathological (the result of an illness or illness process) and is held despite evidence to the contrary. As a pathology, it is distinct from a belief based on false or incomplete information, dogma, stupidity, poor memory, illusion, or other effects of perception. Delusions typically occur in the context of neurological or mental illness, although they are not tied to any particular disease and have been found to occur in the context of many states.

10. Reduplicative Paramnesia

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Reduplicative paramnesia is the delusional belief that a place or location has been duplicated, existing in two or more places simultaneously, or that it has been ‘relocated’ to another site. Taken from the Benson and colleagues study, the following excerpt illustrates some of the core features of the delusion. The patient had suffered a head injury after a fall in his home. The impact had caused a fractured skull. A few days after admission to the Neurobehavioural Center, orientation for time was intact, he could give details of the accident (as related to him by others), could remember his doctors’ names and could learn new information and retain it indefinitely. He exhibited, however, a distinct abnormality of orientation for place. While he quickly learned and remembered that he was at the Jamaica Plain Veterans Hospital (also known as the Boston Veterans Administration Hospital), he insisted that the hospital was located in Taunton, Massachusetts, his home town. Under close questioning, he acknowledged that Jamaica Plain was part of Boston and admitted it would be strange for there to be two Jamaica Plain Veterans Hospitals. Nonetheless, he insisted that he was presently hospitalized in a branch of the Jamaica Plain Veterans Hospital located in Taunton. At one time he stated that the hospital was located in the spare bedroom of his house.

9. Fregoli Delusion

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The Fregoli delusion or the delusion of doubles is a rare disorder in which a person holds a delusional belief that different people are in fact a single person who changes appearance or is in disguise. The syndrome may be related to a brain lesion, and is often of a paranoid nature with the delusional person believing themselves persecuted by the person they believe is in disguise.

8. Morbid jealousy

Morbid jealousy (or Othello syndrome) is a condition in which a person holds a delusional belief that their spouse or sexual partner is being unfaithful. An affected person typically makes repeated accusations of infidelity based on insignificant or minimal evidence, often citing seemingly normal or everyday events or material to back up their claim. They may also take great pains to test their partner’s fidelity, endlessly checking and re-checking mobile phones and diaries, and can go to considerable lengths to monitor their behavior and movements. This may be taken to extremes, such as “private-eye” type surveillance outside of the partner’s residence or workplace, following them into the bathroom in case their partner has an illicit meeting, or even hearing the voice of the perceived lover. Delusional jealousy can occur in either sex, and regardless of an individual’s sexual orientation. Unlike other delusional disorders, delusional jealousy has a strong association with violence and in some cases stalking behavior. At the very least affected individuals tend to be irritable, aggressive, or confrontational.

7. Grandiose Delusions

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Grandiose delusions are characterized by fantastical beliefs that one is famous, omnipotent, or otherwise very powerful. The delusions are generally fantastic, often with a supernatural, science-fictional, or religious bent. Grandiose delusions are distinct from grandiosity, in that the sufferer does not have insight into his loss of touch with reality. In colloquial usage, one who overestimates one’s own abilities, talents or situation is sometimes said to have ‘delusions of grandeur’. This is generally due to excessive pride, rather than any actual delusions.

6. Cotard Delusion

The Cotard delusion is a rare neuropsychiatriccondition in which people hold a delusional belief that they are dead (either figuratively or literally), do not exist, are putrefying, or have lost their blood or internal organs. In rare instances, it can include delusions of immortality. Young and Leafhead describe a modern-day case of Cotard delusion in a patient who suffered brain injury after a motorcycle accident: [The patient’s] symptoms occurred in the context of more general feelings of unreality and being dead. In January 1990, after his discharge from hospital in Edinburgh, his mother took him to South Africa. He was convinced that he had been taken to hell (which was confirmed by the heat), and that he had died of septicaemia (which had been a risk early in his recovery), or perhaps from AIDS (he had read a story in The Scotsman about someone with AIDS who died from septicaemia), or from an overdose of a yellow fever injection. He thought he had “borrowed my mother’s spirit to show me round hell”, and that she was asleep in Scotland.